The first 405 had a straight voltage-dependant-current-limiter. Once you reached the threshold it would reduce the output current as the output voltage increased. This is the behaviour of a negative resistance. If the resistive part of the loudspeaker impedance was below the value of the negative resistance of the 405 in limiting mode what you ended up with was an oscillator. So, the amp would not only clip, it would bounce between the power supply rails. Very nasty sounding. This used to happen with the old pre-aB 104, which used to drop down to under 3 Ohms around 3 kHz. There were also problems with the Cantata.

KEF brought out the 104aB which had a much "nicer" impedance and Quad modified the 405 limiters to have some sort of averaging rather than instant operation in the series 2.

Model 104|2 has close to a pure resistive 4 Ohm input impedance and so there will be no problems with 405 limiter operation. The small heatsink, however, may not be adequate for long term use at full output.
SG

This design is a "tweaked" single amplifier module - AVOID unless they are very cheap ~$200 pair.

You want to look for a pair of competently engineered bridged 405 designs (4010's), where one of the of the inverting amplifiers is inverted to double the current capacity making the original design more suitable for driving 4 Ohm loads._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

Sorry Colin I seem to often end up disagreeing with you. Bridge mode does not make a 405 more suitable for driving low loads (except for heatsink sharing). I will explain:

If you invert the input to one of the amps and use both in bridge mode to drive one speaker you are potentially doubling the VOLTAGE output. The CURRENT limit stays the same (unless you mod the current limiters).

It works like this: if you apply +2 Volts to one end of a 4 Ohm load and -2V to the other end of the load you will get 1A in the load. So each amp is putting out 2V and 1A so they are each "seeing" a 2 Ohm load.

If you want to double the current you have to put the amps in parallel, which they won't like at all unless they are gain matched to perfection including tracking at all outputs and temperatures. A no-no.

In general if an amp is designed for R Ohms you can use it in bridge mode with a 2R load without any current limit problems.

If you look at the current/voltage graphs supplied in the manual by Quad for the stock 405 & 405-2, they are driven into current and voltage limiting at lower RMS output levels with 4 Ohm loads.

Quad produced the parallel bridging kit to obviate this effect for speakers like the Ref 104-2 due to too many failures.
The existing power amplifier modules should be "matched quartets" btw before modification.

A stock 405-2 driving a 4 Ohm resistive load sounds rather "edgy" to my ears (clipping of some kind?).

Having always taken Quad's word at face value due to the novel nature of the design I just trotted out the Quad mantra, I've got all of the Quad paperwork and can photograph the bridging modules if you'd like some copies.

A series modification for 15 Ohm loads was also produced using an input transformer.

The only worthwhile one btw is changing the capacitor in the input signal path to a Black Gate - they weren't available when the 405 was designed, but probably would have been included if they had been .

Finally for those not in the know a F, M or C is a B.B.C. unit; f, m & c are "broadcast standard" and the 606-2 never existed as an official model ._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

Last edited by ColinR on Fri Sep 29, 2006 8:34 pm; edited 1 time in total

i've heard great things about the macmod 405's. many say that the monoblocks sound lightyears beyond stock models. they suck?

i'm just casually looking at this point

My problem is that I can find no information regarding the macmod's performance into 4 Ohm loads so I'd err on the side of caution until someone comes up with the numbers._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

well, i am potentially buying a minty 405-2 and 34 preamp with orginal boxes and manuals. made in 1984 and from the original owner. $250 USD for both with all the DIN connectors and banana plugs.

there's some saying that the 405-2 is overmatched by the 104/2, but i'll give it a whirl. he's coming over tomorrow and he said if it doesnt sound good, i don't have to buy it.

so, here's the question, can this thing, with the necessary mods (new caps, op-amps and removal of the current limiter) rock the 104/2? or am i wasting my time and money (although i'm sure i can sell them to at least get my money back out of them).

Grab them ._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

The KEF 104/2's may not sing (better with monoblocks, 520's or 606's imho), but the Calindas and a few of your other speakers certainly will ._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

i will try them with the calindas too. in your opinion, should i keep farting around with the 104/2 or are they enough of a hassle that i should just sell them and enjoy the calindas with my REL Q100 sub?

the 104/2 look real cool, but if they need uber wattage, like Krell, Naim, musical fidelity ,etc. maybe i should just send them down the road?

blowing $1,500 USD for a suitable amp for speakers i might not keep seems stupid, right?

how would you rate the 104/2 against the 1970's KEFs? those bandpass woofers seem like more trouble than they are worth. and they still don't go below 50Hz. what's the big deal with them?

in your opinion, should i keep farting around with the 104/2 or are they enough of a hassle

I think that you've "played" enough with the 104/2's, you'll have to get into basic crossover design to "improve" on what you've got, so if you cannot live with them, get rid.

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Calindas with my REL Q100 sub

Now that's a "fun" combination .

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the 104/2 look real cool, but if they need uber wattage

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blowing $1,500 USD for a suitable amp for speakers

Yes and yes unless you can borrow a power plant for a week to firm up your decision then you must seriously consider

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sending them down the road

.

I suppose I'm fairly lucky in having a large collection of working/serviced amplifiers and I can borrow bigger birds if required, but then again I'm not an audiophile, I listen to my hi-fi ._________________This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

i mean i don't need the money or anything. i could just put the 104/2 in the garage for now. of course, the impetus for the quad 405-2 was to power the 104/2. but now it sounds like i might buy the quads because they are cool, have lots of potential and would be an interesting point/counterpoint to my tube mcintosh gear.

have you ever had a pair of 104/2 or spent some time with a pair. what did you think of them and how would you compare them to the calindas. so far, the calindas are a blast! the 104/2 always seem to have the potential to sound better, but so far, they leaving me unsatisfied. of course more power would be key. but i simply don't get the need that much current and power for a speaker with 5 drivers that don't go below fricken 50Hz!

at first, i dug the 104/2, but i mistook the sloppy bass for warmth. the calindas sound much more balanced and trasparent.